1. Field of the Invention
This invention relates to an optical signal reading device for an optical information recording and reproducing apparatus having at least either the function of reading information recorded on the track of a disc-like record carrier or the function of optically recording information on the recording medium, and more particularly it relates to an optical signal reading device having a beam splitter of improved construction.
2. Description of the Prior Art
Conventional devices of this type include a video disc device and a digital audio disc device. Conventional devices of this type generally comprise a light source, an objective for focusing the light emitted from the light source on the disc surface as a spot of light, and a light detector for detecting the reflected light from the disc surface led in through said objective. A beam splitter is disposed between the objective, light source, and light detector. Examples of such arrangements are disclosed in U.S. Pat. 4,261,646 and U.K. patent specification 1,532,345.
In these prior art devices, the light beam from the light source falling on the beam splitter is approximately at right angles to the reflected light from the disc surface guided to the light detector through the beam splitter. However, as fully described below, with such arrangement there are problems that installation of the light detector and beam splitter requires high precision and that complex adjustments of these optical parts are required.
FIG. 1 is a schematic structural view of a conventional optical system for explaining the aforesaid problems. Referring to FIG. 1, the light emitted from a light source M is transmitted through the reflecting surface of a beam splitter 1 and then through an objective 2 and is focused on a disc surface 3. The reflected light from the disc surface 3 is guided by the objective 2 to the beam splitter 1. It is reflected by the reflecting surface of the beam splitter 1 and transmitted through a cylindrical lens 4 to a light detector N. In such arrangement, the distance between the beam splitter 1 and the light source M is equal to the distance between the beam splitter 1 and the convergence point A of light rays reflected by the beam splitter 1 to the light detector N. In other words, the point A and the light source M are in conjugate relation to each other with respect to the beam splitter 1. Where such astigmatic optical system is used, the light detector N must be located at a fixed distance from the convergence point A. In such arrangement, however, if the beam splitter is not correctly located, the position of the light detector N must be correctly readjusted. Such a problem may be more clearly understood by reference to FIG. 2 showing, on an enlarged scale, the relation between the beam splitter, light source, and light detector. The light emitted from the light source M is guided through the point O on the reflecting surface of the beam splitter 1 to the objective (not shown in FIG. 2). On the other hand, the light guided by the objective and reflected by the disc surface is reflected at point O on the reflecting surface of the beam splitter 1 and transmitted through the cylindrical lens 4 to the light detector N. In FIG. 2, as is clear from the above description, OM=OA and it is necessary that the distance NA=l between the convergence point A and the light detector N be constant. If the beam splitter 1 is downwardly deviated as shown in phantom lines in FIG. 2, however, the reflected light from the objective is reflected at point S on the reflecting surface of the beam splitter 1. Therefore, the convergence point of the reflected light from point S moves to point A1 which satisfies the relation SM=SA1. The distance SM between the point S on the reflecting surface of the beam splitter 1 and the light detector N is longer by SO than before the beam splitter 1 is deviated. However, the distance SN between the point S on the reflecting surface of the beam splitter 1 and the light detector N is equal to the distance ON between the point O on the reflecting surface of the beam splitter 1 before the latter is deviated and the light detector N. Therefore, the distance between the light detector N and the convergence point A1 is longer than 1 by SO, making it necessary to readjust the light detector N to a position N1 shown in phantom line. Thus, in the method disclosed in the prior art, since the reflected light from the disc surface guided from the beam splitter 1 to the light detector is approximately at right angles to the light emitted from the light source M to the beam splitter 1, it has been impossible to avoid the drawbacks that the beam splitter must be very accurately positioned and that complex adjustments of the light detector N are required.